1. Field of the Invention
The invention relates to Group 4 metal-containing organosilicon dendrimers, methods of preparation thereof, and methods of use thereof.
2. Description of the Prior Art
Dendrimers are chemical compounds characterized by a regular, highly branched structure as shown schematically in FIG. 1. Dendrimer 10 of FIG. 1 is a second generation dendrimer, denoted by a generation number, G.sub.n, equal to G.sub.2. Dendrimer 10 includes an initiator core 12 from which branches, whose number is denoted by N.sub.n and characterized by length l, emanate. Four main branches (N.sub.1 =4) emanate directly from initiator core 12 and form four primary branch points 14 from each of which three new branches (N.sub.1 =3) emanate and form secondary branch points 16 as the next generation polymer is produced. Branches that emanate from branch points 16 terminate in end groups 20.
Dendrimers are ideally monodisperse, i.e., they consist of single, identical species, all of which have the same composition and molecular weight. Dendrimers can be characterized by a range of molecular weights, ranging from relatively low oligomeric molecular weight, to relatively high polymeric molecular weights. Dendrimer molecular weights can depend on several factors including length of the arms, the extent of arm branching, the functionality of the branching groups in the arms, the length of connecting groups between branching sites and the functionality of the dendrimer core. Typically, dendrimers are soluble in organic solvents and their solubility in a particular solvent can be optimized by the choice of appropriate functional groups for the end groups. However, end groups may be chosen so as to result in water solubility. Dendrimers of intermediate generation number, G.sub.n, typically with n in the range of from about 1 to about 10, depending upon the dendrimer system, are characterized by an uncongested periphery with empty space between neighboring end groups. As such, intermediate generation number dendrimers have high surface areas and a relatively large proportion of unoccupied dendrimer interior volume.
Dendrimers can be synthesized using a "divergent procedure", according to which dendrimers are grown outward by repetitive chemical steps using a multifunctional central core molecule as the starting material. According to this approach, successive hydrosilylation and vinylation, or alternatively, allylation, steps are performed on the polyfunctional core, which can be a tetravinylsilane or tetrallylsilane to form the dendrimeric structure. The divergent procedure is reliable and effective, provided that appropriate reaction conditions are maintained until that point in the synthesis when steric congestion at the dendrimer periphery hinders further dendrimer growth. However, the divergent procedure is multistep, relatively expensive and, hence, may not ideally suited for large scale commercial applications.
Alternatively, a "convergent procedure" can be used to synthesize dendrimers by preparing segments of the dendrimer first and then attaching the segments to a central core molecule. However, the convergent procedure is also multistep, relatively expensive and, hence, may not be ideally suited for large scale commercial applications.
A third approach for dendrimer synthesis is a "cascade procedure". According to this approach, a single monomer is used in a single type of reaction to prepare the dendrimer. Generally, a cascade procedure begins with a monomer containing two different reactive functions, x and y, more specifically, one x function and two or more y functions. The x and y functions are selected so that x can react with y, but not with itself. The reaction between x and y is initiated so that branching growth occurs to produce a dendrimer. The structure of the dendrimers produced according to a cascade procedure will not be as regular a structure as that of dendrimers synthesized according to a divergent or convergent procedure. Dendrimers produced using the cascade procedure will be polydisperse rather than monodisperse and have been referred to as "hyperbranched" materials. Typically, the cascade procedure is relatively inexpensive and well-suited to large scale commercial applications.
A type of organosilicon dendrimers, carbosilane dendrimers, and their preparation are described in Seyferth et al., Organometallics, 13 (1994) 2682-2690. A typical dendrimer prepared in the foregoing study is shown in FIG. 2.
Thus, it would be highly desirable to exploit the foregoing dendrimer characteristics, including relatively high surface area and relatively high porosity, for applications including catalysis by preparation of dendrimers having end or interior group substituents with a desired chemical activity. Moreover, it would be highly desirable to exploit the foregoing desirable cascade procedure characteristics for the synthesis of organosilicon dendrimers.